Process for the recovery of tantalum and niobium and other metals from tin slag

ABSTRACT

Aluminum oxide or calcium oxide is added to tin slag, which contains a relatively high silica content along with a source of carbon, and the mixture is fused. The added aluminum oxide or calcium oxide combines with the silicon oxide and other metallic compositions present to form a slag phase of a new compositiom which has a melting point 100* - 400* C. higher than the original slag composition. In the fusion the carbon reduces the tantalum and niobium and other oxides to the respective metals and the higher melting range of the new slag composition results in super heating of the metal alloy which facilitates a more effective ultimate separation of the metal alloy.

United States Patent [1 1 in] 3,923,497 Scott *Dec. 2, 1975 1 PROCESS FOR THE RECOVERY OF 3,300,297 1/1967 Fields 75/24 TANTALUM AND NIOBIUM AND OTHER g; g lx i2 us 18011 METALS FROM TIN SLAG 3,811,867 5/1974 Scott 75/24 [75] Inventor: John J. Scott, Niagara Falls, Canada [73] Assignee: Norton Company, Worcester, Mass.

I Notice: The portion of the term of this patent subsequent to May 21, 1991, has been disclaimed.

[22] Filed: Mar. 21, 1974 [211 Appl. No.: 453,275

Related U.S. Application Data [63] Continuation-impart of Ser. No. 297,595, Oct. 16,

1972, Pat. No, 3,811,867.

[52] U.S. Cl. 75/24; 75/1; 75/133.5 [51] Int. Cl. C22B 51/00 [58] Field of Search 75/1, 24, 133.5

[56] References Cited UNITED STATES PATENTS 2,438,911 4/1948 Gronningsaeter 75/24 X Primary ExaminerPatrick P. Garvin Assistant Examiner.lohn F. Niebling Attorney, Agent, or Firm-Arthur A. Loiselle, Jr.

[57] ABSTRACT Aluminum oxide or calcium oxide is added to tin slag, which contains a relatively high silica content along with a source of carbon, and the mixture is fused. The added aluminum oxide or calcium oxide combines with the silicon oxide and other metallic compositions present to form a slag phase of a new compositiom which has a melting point 100 400 C. higher than the original slag composition. In the fusion the carbon reduces the tantalum and niobium and other oxides to the respective metals and the higher melting range of the new slag composition results in super heating of the metal alloy which facilitates a more effective ultimate separation of the metal alloy.

2 Claims, No Drawings PROCESS FOR THE RECOVERY OF TANTALUM AND NIOBIUM AND OTHER METALS FROM TIN SLAG This is a continuation-in-part of Application Ser. No. 297,595, filed Oct. 16, 1972 now U.S. Pat. No. 3,811,867.

BACKGROUND OF THE INVENTION The invention relates to the oresmelting art. More specifically it relates to the arc furnace reduction of tin slag particularly for the ultimate production of tantalum and niobium, but equally effective for the extraction of other metals.

Tin slag has long been known as a source of the relatively scarce metals tantalum and niobium. Common sources for such tin slag are the Congo, Malaya and Thailand.

Various techniques are known for the extraction of tantalum and niobium and other metals present from tin slags. For example, the process described in U.S. Pat. No. 2,131,350 wherein tantalum and niobium are removed from ores or metallurgical by-products like tin slag via a carbothermic reduction of the niobium and tantalum compounds in these materials. The impurities in the ores are selectively reduced and removed leaving a concentration of niobium and tantalum slag. The niobium and tantalum slag must then be further processed to reduce these to a mixed metal alloy, from which the metals niobium and tantalum per se are removed by additional processing. The main disadvantage of this approach is the extensive processing required which is economically impractical.

In another method described in U.S. Pat. No. 2,972,530 niobium and tantalum bearing ores or concentrates are directly smclted in an arc furnace. Iron is added to the ore or concentrate in order to form an alloy of niobium, tantalum, and iron which will be molten at the temperature of the melt. This facilitates separation of the niobium and tantalum bearing alloy, however, it presents the disadvantage of having an inordinately large amount of iron which must be removed.

A third approach is that described in the United States Department of Interior, Bureau of Mines, Report of Investigations No. 6734, entitled Electric-Arc Furnace Reduction of Tin Slag for Production of Columbium-Tantalum-Bearing Alloy," authored by W. L. Hunter and O. C. Fursman, 1966. Using the method described therein 90 percent of the columbium (niobium) and 85 percent of the tantalum contained in tin slag were recovered by an intermittently operative process for direct smelting in, for example, steel making furnaces. Char, metallurgical coal, wood chips, charcoal, and graphite were used as reducing agents in the main. Additions of silicon metal appeared to enhance the yield of tantalum and columbium (niobium) alloy therefore it was used as a reducing agent in conjunction with the carbon. Because the depleted slag was acidic in composition small amounts of lime were apparently added in one test to minimize slag attack on the basic furnace refractory. In general, the tin slag with carbon rcductant was charged to the furnace, and successive batches of the mixture were fused and the temperature raised to 1,500 1600 C. holding the material in the furnace until reduction was complete, a tantalumniobium alloy settled to the bottom of the furnace and the remainder of the molten depleted slag was removed from the furnace. The tantalum-niobium alloy remained deposited on the furnace hearth as a plastic mass as the unwanted slag was tapped and the alloy was ultimately physically removed, for example by raking, following the final slag tap. Despite the high yield of tantalum and niobium which is produced by this process it suffers from -the serious shortcoming which results from the necessary intermittent operation of the furnace because of the necessity of raking the alloy from the bottom of the furnace which in turn results directly from the relatively low temperature of the bath which never gets hot enough to reduce the viscosity of the collected tantalum-niobium alloy to permit tapping thereof.

SUMMARY OF THE INVENTION Not including the oxides or iron, tantalum, and niobium, tin slag is principally made up of calcium oxide, aluminum oxide, silicon oxide, with varying lesser amounts of titanium oxide, manganese oxide, magnesium oxide, tungsten oxide, and zirconium oxide depending on the geographical origin of the tin ore from which the bulk of the tin has been removed, which tin melting process produces the slag which forms the raw material for the present process. Thus the tin slag may be considered essentially as a calcium oxide-aluminum oxide-silicon oxide system. A study of the calcium oxide-aluminum oxide-silicon oxide phase diagram of E. F. Osburn and A. Muan (see Phase Diagrams for Ceramists, published by the American Ceramic Society, 1964, page 219, FIG. 630) shows that the addition of either calcium oxide, or aluminum oxide, or mixtures of aluminum oxide and silicon oxide, or mixtures of calcium oxide and silicon oxide, or mixtures of aluminum oxide and calcium oxide, greatly affects the melting point of the calcium oxide-aluminum oxide-silicon oxide system of the original tin slag, by increasing the melting point thereof, depending on how much of each material is added to the starting calcium oxidealuminum oxide-silicon oxide mixture. From the same diagram it can be seen that if the silicon oxide content alone is increased, just the reverse occurs to the melting point, i.e., it is decreased.

The present invention is an improved method for extracting niobium and tantalum particularly from tin slag but the other metals present may also be more easily recovered if desired. The improvement results directly from the increase in melting point of the calcium oxide-aluminum oxide-silicon oxide phase of the slag which results from the addition to said slag of specific additional amounts of either' calcium oxide, aluminum oxide, or mixtures of these. When the so modified tin slag is heated to its elevated melting point, it causes superheating of the iron-niobium-tantalum alloy which is formed. The superheating of the metal alloy in turn facilitates more effective separation of the alloy from the remainder of the slag. The separation of the alloy, it is believed, takes place in two steps or phases. While the slag is molten and the reduction of the oxides of the metals of interest has taken place, the metals form the alloy in small droplets. The droplets increase in size until they gradually fall through the melt to the bottom of the furnace and collect there in a pool. At any given time there is more or less metal alloy in globule or droplet form suspended in the melt. If the melt is allowed to remain molten for a sufficient time, the alloy collects in a pool in the bottom of the furnace. Because the alloy pool is superheated, its viscosity is low enough 3 that the alloy may be readily tapped or poured from the furnace whereby a continuous extraction process results. The alloy depleted slag may be allowed to solidify, crushed up, and treated on 21 Sutton Table and/or passed through a magnetic separator to remove those globules of alloy still suspended in the slag.

The present invention may also be practiced in a batch type fusion furnace, commonly referred to as a Higgins furnace, such as that described in U.S. Pat. No. 775,654, rather than the tapping furnace described above. When the batch type furnace is utilized, an appropriate amount of carbon and calcium oxide, aluminum oxide, or a mixture of the two, are added to the original slag. The modified slag is gradually added to the furnace with the electrodes in operation in the manner normal for this type of furnace; as each increment of modified slag is added it becomes fused and the oxides of niobium, tantalum and iron are reduced to the corresponding metals. When the furnace run is completed, i.e., the furnace is filled, the power is shut off, the electrodes removed and the molten mass in the furnace allowed to cool. The resulting pig includes a small frozen pool of alloy on the bottom thereof, but the preponderance of the alloy is distributed throughout the pig in the form of globules ranging in size of from approximately 6 mesh down to 325 mesh. The pig is then broken up and impact crushed. The crushed material is then passed over a Sutton Table and/or a magnetic separator which removes the iron-niobium-tantalum alloy globules from the crushed slag particles.

A sufficient amount of the aforementioned calciumoxide, aluminum oxide, silicon oxide combinations must be added to increase the melting point of the molten mass of the calcium oxide-aluminum oxide-silicon oxide phase of the tin slag at least approximately 100 C. to make the reduced alloy flowable, and to enhance its tendency to form into globules in the molten slag. As can be seen from the phase diagram, such manipulations of the calcium oxide and aluminum oxide additions can produce as great an increase in'the melting point of the calcium oxide-aluminum oxide-silicon oxide system of as much as 400 C. The specific quantity of calcium oxide or aluminum oxide which is to be added to the tin slag to generate a given increase in melting point, does of course depend on the composition of the particular tin slag being employed. In view of the present disclosure, it becomes an easy matter for one skilled in the art to select the proper amounts of calcium oxide, aluminum oxide, or a mixture of these additives once the chemical analysis of the tin slag is known. The calcium oxide-aluminum oxide-silicon oxide phase diagram mentioned above can be consulted to enable the operator to design a mixture which will melt at a temperature sufficiently high to reduce the viscosity of the molten alloy to a degree making it possible to tap the furnace and remove it in a fluid state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the preferred practice of the present invention, an addition of either relatively pure aluminum oxide or calcium oxide is most desirable but mixtures may be used. There are sources of these materials that are less than pure which are operable, for example bauxite as a source of aluminum oxide, and limestone as a source of calcium oxide.

Furthermore, from the calcium oxide-aluminum oxide-silicon oxide phase diagram it can be seen that an increase in temperature of the calcium oxide-aluminum oxide-silicon oxide phase of the tin slag can be facilitated by mixtures of aluminum oxide and calcium oxide or even aluminum oxide combined with small amounts of silicon oxide as in bauxite. These are, however, less desirable than straight additions of aluminum oxide or calcium oxide.

The following are illustrative of the preferred practice of the invention and are in no way to be construed as limiting. In the examples a niobium-tantalum alloy button with small amounts of other metals present was formed. It should be understood, however, with respect to Examples I and II, that instead of cooling the furnace, aluminum oxide or calcium oxide enriched slag could have been poured off and additional enriched tin slag added and fused, this being repeated until a substantial amount of molten alloy was generated, at which point the feeding of slag to the furnace would be stopped and the niobium-tantalum tapped off. But the furnace need not be cooled and after tapping the metal phase, the upgrading process can be continued indefinitely with periodic tapping of the liquid metal alloy phase.

EXAMPLE I A mixture of 62.5% Thailand tin slag, 31.25% calcium oxide, 6.25% carbon was fed to an electric arc furnace operating at an average KWl-I of 51.8 for a period of 0.717 of an hour. The original tin slag had the following analysis:

Ta o 10.69% Nbp, FeO 6.68% (Fe,0;)

SnO TiO, 10.99% SiO, 20.14% A1 0 12.99% CaO 1 6. l 0% M g0 W0; ZrO, 3 .2 l

Rare Earth Oxides From this furnace 55 pounds of slag analyzing 17.5% SiO 61.5% CaO, 21% A1 0 was formed, ignoring the other metallic compounds present. The melting point of this newly formed slag, as calculated from the phase diagram, was approximately l,750 C. Approximately one-third of the tantalum present in the slag was found in the metal alloy that was allowed to form into a metal button in the bottom of the furnace but which could have been tapped in its liquid phase if desired. This button analyzed:

Weight 2 46.75 Fe 17.09 Ta 5.5 Ti 3.47 Carbon .85 Si .47 Al The percent of the other elements was not determined. Based on a phase diagram analysis of the principal metals in the button, the melting point of the mixture was below 1,500 C.

EXAMPLE u A mixture of 58.8% Thailand tin slag (same as in Example I), 29.4% A1 0 5.9% SiO 5.9% carbon was fed to an electric arc furnace operating at an average KWH of 51.5 for a period of 0.583 of an hour. The CaO.Al- 0 .SiO phase of this slag and additives mixture was SiO,,, 6% CaO and 79% A1 0 the melting point of which is approximately l,850 C. as determined on the phase diagram. A metal button was allowed to form in the furnace, which contained approximately 43% of the tantalum originally present in the slag, and analyzed:

Weight 71 51.39 Fe 9.4 Ta 1.74 Ti 1.74 Carbon 1.39 Si The percentage of other elements was not determined. As in Example 1, a phase diagram analysis showed the melting point of the mixture to be below l,500 C.

EXAMPLE [[1 A 1260 pound mixture of 71.4% by weight of Thailand tin slag, 14.3% by weight of calcium oxide, and 14.3% by weight of barley coal was fused in the conventional manner in a 1,500 pound Higgins type furnace over a period of 4.58 hours. The contents of the furnace was then cooled, broken up, and finally impact crushed. The fusion process resulted in a substantial quantity of Fe-Ta-Nb alloy collecting in the bottom of the ingot, however, most of the alloy was distributed as globules, throughout the ingot. The crushed material was then treated on a Sutton Table which separated the denser alloy globules from the slag particles. The total tantalum contained in the original slag was 82.9 pounds. The tantalum contained in the Fe-Ta-Nb alloy which was produced, was 59.1 pounds for a 71.2% recovery of the available tantalum, in the original tin slag.

The resulting metal alloy may then be separated into its constituent metals by any of the known prior art methods. For example, the metal alloy may be depleted of iron by leaching with hydrochloric acid and the tantalum and niobium separated from each other as described by May et al., in Separation of Tantalum From Columbium by the Hydrofluoric Acid-Sulfuric Acid- Methyl lsobutyl Ketone System," Bureau of Mines Report of Investigation 5862, 1961.

It is further understood that the reducing reaction which takes place in the molten bath can be improved in terms of the efficiency thereof, by the addition of metals such as aluminum or calcium. For example, aluminum oxide rich in aluminum would enhance the reduction reaction if it was added as the aluminum oxide source discussed above. In addition, the basic process 6 may be utilized by fusing the modified slag in a tapping type furnace, as in Examples I and 11 above, but instead of operating the process on a continuous basis, the melted slag may be cast into ingot molds and allowed to solidify. The relatively small cast ingots may then be crushed and treated magnetically or on a Sutton Table as described in Example Ill.

What is claimed is: l. A process for the recovery of tantalum and niobium and other metals from commercial tin slag by the reduction of the oxides of said metals, by fusion of the tin slag in the presence of a reducing agent, wherein the improvement comprises:

adding to said tin slag a sufficient amount of a material selected from the group consisting of aluminum oxide, calcium oxide, silicon oxide-aluminum oxide, silicon oxide-calcium oxide, and aluminum oxide-calcium oxide to produce a molten bath having principally a CaO.Al O .SiO phase that melts at a temperature to 400 C. greater than the Ca0- .Al O .SiO phase of said tin slag, said amount being determined by reference to a CaO.Al O SiO phase diagram; fusing the'mixture of tin slag and additional aluminum oxide, calcium oxide, silicon oxide-aluminum oxide, silicon oxide-calcium oxide, or aluminum oxide-calcium oxide, in a tapping type are furnace, periodically tapping off the tantalum-niobium depleted molten slag and replacing said tapped slag with a further addition of said mixture, repeating said slag tapping and mixture addition steps until a substantial amount of tantalum-niobium containing alloy has accumulated in the bottom of said furnace, and tapping said alloy from the bottom of the furnace.

2. A process for the recovery of tantalum and niobium and other metals from commercial tin slag by the reduction of the oxides of said metals, by fusion of the tin slag in the presence of a reducing agent, wherein the improvement comprises:

adding to said tin slag a sufficient amount of a material selected from the group consisting of aluminum oxide, calcium oxide, silicon oxide-aluminum oxide, silicon oxide-calcium oxide, and aluminum oxide-calcium oxide to produce a molten bath having principally a CaO.Al O .SiO phase that melts at a temperature 100 to 400 C. greater than the Ca0- .Al O .SiO phase of said tin slag, said amount being determined by reference to a CaO.Al O SiO phase diagram; fusing the mixture of tin slag and additional aluminum oxide, calcium oxide, silicon oxide-aluminum oxide, silicon oxide-calcium oxide, or aluminum oxide-calcium oxide in a batch type are furnace, cooling the contents of said furnace, breaking up and crushing the resulting ingot, and separating the tantalum-niobium alloy in globular form, from the remainder of the slag. 

1. A PROCESS FOR THE RECOVERY OF TANTALUM AND NIOBIUM AND OTHER METALS FROM COMMERICAL TIN SLAG BY THE REDUCTION OF THE OXIDES OF SAID METALS, BY FUSION OF THE TIN SLAG IN THE PRESENCE OF A REDUCING AGENT, WHEREIN THE IMPROVEMENT COMPRISES: ADDING TO SAID TIN SLAG A SUFFICIETN AMOUNT OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE, CALCIUM OXIDE, AND ALUMINUM OXIDE-ALUMINUM OXIDE, SILICON OXIDE-CALCIUM OXIDE, AND ALUMINUM OXIDE-CALCIUM OXIDE TO PRODUCE A MOLTEN BATH HAVING PRINCIPIALLY A CAO.AL2O3.SIO2 PHASE THAT MELTS AT A TEMPERATURE 100*C TO 400* C. GREATER THAN THE CAO.AL2O3.SIO2 PHASE OF SAID TIN SLAG, SAID AMOUNT BEING DETERMINED BY REFERENCE TO A CAO.AL2O3.SIO2 PHASE DIAGRAM, FUSING THE MIXTURE OF TIN SLAG AND ADDITIONAL ALUMINUM OXIDE, CALCIUM OXIDE, SILICON OXIDEALUMINUM OXIDE, SILICON OXIDE-CALCIUM OXIDE, OR ALUMINUM OXIDE-CALCIUM OXIDE, IN A TAPPING TYPE ARC FURNACE, PERIODICALLY TAPPING OFF THE TANTALUM-NIOBIUM DEPLETED MOLTEN SLAG AND REPLACING SAID TAPPER SLAG WITH A FURTHER ADDITION OF SAID MIXTURE, REPEATING SAID SLAG TAPPING AND MIXTURE ADDITION STEPS UNTIL A SUBSTANTIAL AMOUNT OF TANTALUM-NIOBIUM CONTAINNG ALLOY HAS ACCUMULATED IN THE BOTTOM OF SAID FURNACE, AND TAPPING SAID ALLOY FROM THE BOTTOM OF THE FURNACE.
 2. A process for the recovery of tantalum and niobium and other metals from commercial tin slag by the reduction of the oxides of said metals, by fusion of the tin slag in the presence of a reducing agent, wherein the improvement comprises: adding to said tin slag a sufficient amount of a material selected from the group consisting of aluminum oxide, calcium oxide, silicon oxide-aluminum oxide, silicon oxide-calcium oxide, and aluminum oxide-calcium oxide to produce a molten bath having principally a CaO.Al2O3.SiO2 phase that melts at a temperature 100* to 400* C. greater than the CaO.Al2O3.SiO2 phase of said tin slag, said amount being determined by reference to a CaO.Al2O3.SiO2 phase diagram; fusing the mixture of tin slag and additional aluminum oxide, calcium oxide, silicon oxide-aluminum oxide, silicon oxide-calcium oxide, or aluminum oxide-calcium oxide in a batch type arc furnace, cooling the contents of said furnace, breaking up and crushing the resulting ingot, and separating the tantalum-niobium alloy in globular form, from the remainder of the slag. 